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Old 17th May 2008, 03:17 PM   #1
pjanda1 is offline pjanda1  United States
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Default tube rectifiers and electrolytic reforming w/ variac

I've been doing some searches looking for info about electrolytic reforming. I've got a variac and two pieces of old tube rectified gear (one a PA amp and the other an old Conn strobotune musical instrument tuner). I've always heard you can just "bring it up slowly" on variac, but many folks claim that you can hurt your tube rectifier. I've read several different methods, and some of them are moderately complicated. What are the odds of hurting the rectifier tube by just pulling the tubes and plugging the units into a variac? At the least, it sounds as if it would be best to start off around 50% voltage to get the rectifier going.

pj
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Old 17th May 2008, 03:42 PM   #2
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There are several good articles on capacitor reforming that you can Google. I would suggest replacing them to be honest because that is the surest and safest. If your bent on saving a buck then try reforming. I personally haven't had much luck.
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Old 17th May 2008, 06:23 PM   #3
pjanda1 is offline pjanda1  United States
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By "I've been doing some searches," I meant that I've read several good articles on capacitor reforming. All of them dictate better methods than hooking the amp up to a variac, however, that's what many folks talk about doing. My question isn't so much, "what's the best way", but rather, is there an easy way with a decent chance of working or, if I crank it up to 60v over the course of a few seconds and leave it there for 12 hours or so, what are the chances it will destroy the rectifier tube?

I'd like to get the units running as cheap as possible. If they have some potential, I'll certainly replace the 'lytics to ensure long lives.

pj
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Old 17th May 2008, 06:59 PM   #4
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Quote:
what are the chances it will destroy the rectifier tube?
Very good if your cap is shorted. Most seem to dry out and become ineffective but I've seen one that was a dead short and made the 5U4 glow red. What will new caps cost you, $10 or so? Caps that old have really gone past their usable lifespan and trying to reform them is just prolonging the inevitable.

If you still want to reform the caps then you must somehow limit the current through a resistor and keep a constant watch on the leakage current.
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Old 17th May 2008, 07:40 PM   #5
pjanda1 is offline pjanda1  United States
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Perhaps the 'ol dim lightbulb to limit current? Is the low voltage on the rectifier heater an issue?

I'm behind schedule today anyway. In the midst of other stuff, I've been working on hooking up the old variac. It's a surplus 240V 10A Staco that a kindly gent gave me via this board. It's mounted on a U channel base (open back, I really need to build a box for it), and initially I was reading about 50V between the chassis and earth with my DMM (regardless of output voltage). I can't for the life of me figure out why. I then connected the chassis to my earth terminal (a good precaution anyway, I figure) and it didn't blow the newish GFI on the outlet. As I can't find any short (chassis to earth read 0 ohms on the DMM prior to installation of the safety ground), I'm wondering what's going on.

pj
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Old 17th May 2008, 07:50 PM   #6
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I just picked up a Hammond M3, and the junk dealer selling it was
gonna fire it up for me (or whoever). I told him Please NO!!!! I'm
the one buying it, don't want it turned on till I do some tests first.

M3 now safe in my neighbor's garage, you suggest Variac? (I don't
have one). Or worth the effort to form and test those old caps out
of circuit, one at a time? ( I can probably manage that trick, but time
consuming...)

I got a Hammond M3 schematic in my archives, just have to find it...
What are the common failures to watch for?

This thing looks in good shape except the "expression pedal" is
missing the pivot bolt through the center... I probably used the
wrong word to describe which pedal, the one on the right.

Where (if any) do I look to see the connector for an external Leslie?
Not look to buy a connector, but look where on the organ???

All the tubes are here, I have an emission tester to check them...
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Old 17th May 2008, 07:50 PM   #7
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You can't use a variac very effectively with a tube rectifier to bring the voltage up slowly. The reason is that the rectifier doesn't function until the AC voltage is pretty high. And by the time the rectifier sees enough voltage to start to conduct, it will be passing a lot more voltage than you'd want to have applied to old suspect caps, etc. downstream.

You won't hurt your tube rectifier by trying to bring the voltage up slowly on a variac, but the components downstream will not get the benefit of the slow increase in voltage.
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Old 17th May 2008, 07:52 PM   #8
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What about SS rectifiers and a Variac?
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Old 17th May 2008, 07:57 PM   #9
pjanda1 is offline pjanda1  United States
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Sounds like I should just suck it up and replace the caps. At least I won't have to worry about the variac killing me. Though I may read up and see if I can use SS diodes up to a point and then toss the rectifier back in.

I had an M3, but it was in pretty rough shape. I found it at an auction when I was an undergrad. After I left the state, my dad wanted it out of his garage. I pulled the amp from the tone cabinet and sold it here. Pretty much everything else was burned in a ditch. Sad.

pj
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Old 17th May 2008, 07:59 PM   #10
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Radiodaze sells plug in converters that change over to solid state rectifiers without modifying the amp (assuming that the amp can take the higher voltage). For your application I think that they would be perfect. Get one for each of the common rectifier pin outs and you would be set for checking out any amp that might come your way.

mike
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